Implantable cardiac stimulation devices, such as Cardiac Resynchronization Therapy (CRT) devices are commonly used devices to treat cardiac arrhythmias. In general, these devices include an implantable control unit and a plurality of leads. The control unit has sensors and the leads can also function as sensors thereby allowing the control unit to receive signals indicative of the performance of the patient's heart and other parameters indicative of the patient's current physiologic state. Upon detecting the occurrence of a particular arrhythmia, an appropriate electrical therapy can be provided via the leads. In some instances, the appropriate therapy includes providing high voltage waveforms to the heart to terminate defibrillation or cardioversion in one or more of the chambers of the heart. In other instances, lower voltage pacing pulses are provided to one or more chambers of the heart to induce the heart to beat in a more regular fashion.
Over time these types of implantable cardiac stimulation devices have become increasingly more capable. One example of which is a CRT device that provides pacing pulses potentially to all four chambers of the heart in an effort to resynchronize the beating of the heart between the left and right hand sides of the heart and also between atria and ventricles. As is understood, after ischemic events, heart tissue may become damaged and electrical conduction within the chambers of the heart may be impeded. By positioning electrodes in both the left and right sides of the heart, for example in both the left and right ventricles of the heart, the heart can often be induced to beat in a more synchronized fashion thereby enhancing the hemodynamic performance of the heart.
One difficulty that occurs is that the tissue within the left ventricle is known to often have poor cardiac conduction. It is often dead or damaged after an ischemic event or may be otherwise diseased. When the tissue is dead, damaged or diseased, conduction within the tissue may be impeded thereby inhibiting the propagation of the electrical impulses which would cause the left ventricle to contract in a normal fashion. Additionally, the innervation of the muscular tissue of the heart can be blocked or damaged. In these cases, even though the muscular tissue is able to conduct normal, the pathway for the signals to synchronize the contraction of the tissue is damaged. Both cases can result in a contraction that is not appropriately synchronized, and has a compromised mechanical output.
To address this particular issue in the context, bi-ventricular pacing is used. The biventricular pacing uses multiple electrical foci to deliver electrical stimulation to the heart. It is believed that multiple left ventricular leads are better able to promote synchronous mechanical contractions. These contractions can be generated through synchronous multisite depolarization of the left ventricle. The multiple sites of stimulation can overcome the effects of dead, damaged or diseased tissue. This can result in the damaged tissue being bypassed resulting in more normal contractions of the heart.
While multiple leads may promote better therapy for a damaged heart, applying electrical stimulation to multiple leads results in an increase in the amount of energy being expended by the implanted cardiac stimulation device. As is understood, implanted cardiac stimulation devices generally are power limited, typically using a battery for energy storage. Increased energy consumption results in reduced device life expectancy. Depleted energy supplies can be dangerous to the patient as a result of the device being unable to continue to deliver therapy or, at a minimum, may require an invasive surgical procedure to replace the energy supply.
Generally, therapeutic pulses are delivered to heart tissue between an anode and a cathode of an electrical system. Generally, stimulation that results in depolarization of the heart tissue is provided via the cathode for cathodic stimulation. Applying cathodic stimulation in this manner to one additional electrode of an implanted cardiac stimulation device at, for example, a rate of 60 beats per minute, may result in the loss of one year of life from an exemplary intracardioverter defibrillator (ICD) or two years from an exemplary pacemaker.
Thus, while it is desirable to be able to provide stimulation pulses to a plurality of electrodes implanted within the heart, the more stimulation pulses that are provided the greater the drain is on the battery of the implanted cardiac stimulation device. Hence, there is a need for a process by which multiple electrodes implanted within a patient's heart can be stimulated but done so in such a manner that reduces the consumption of limited battery power.